The present invention relates generally to semiconductor fabrication, and more particularly to methods for fabricating improved ultra-large scale integration (ULSI) semiconductor devices such as ULSI metal oxide silicon field effect transistors (MOSFETs).
Semiconductor chips or wafers are used in many applications, including as processor chips for computers, and as integrated circuits and as flash memory for hand held computing devices, wireless telephones, and digital cameras. Regardless of the application, it is desirable that a semiconductor chip hold as many circuits or memory cells as possible per unit area. In this way, the size, weight, and energy consumption of devices that use semiconductor chips advantageously is minimized, while nevertheless improving the memory capacity and computing power of the devices.
A common circuit component of semiconductor chips is the transistor. In ULSI semiconductor chips, a transistor is established by forming a polysilicon gate on a silicon substrate and separated therefrom by a gate insulator. Source and drain regions are then formed in the substrate beneath the gate by implanting appropriate dopant materials into the areas of the substrate that are to become the source and drain regions. This generally-described structure cooperates to function as a transistor.
Importantly, as mentioned above, the gate must be insulated from the source and drain regions by a thin gate insulator, typically a layer of oxide. As the size of the transistors is reduced by ULSI technology, the thickness of the gate insulator becomes ever smaller. Owing to the relatively high electric field across the gate insulator layer, however, charge carriers undesirably can tunnel across the gate insulator layer, particularly when the gate insulator layer is made very thin as it is in ULSI technology. This renders the transistor xe2x80x9cleakyxe2x80x9d, degrading its performance.
To alleviate this problem, high-k dielectrics (dielectrics that have high dielectric constants) are used as the gate insulator. Unfortunately, undesirable chemical reactions can occur between the high-k gate insulator and the conventional polysilicon gate electrode above the gate insulator. For this reason, it can be advantageous to use metal gate electrode materials in place of or in addition to the polysilicon when high-k material is used as the gate insulator.
The present invention recognizes, however, that a metal electrode that is sufficient for establishing a gate of a P-channel MOSFET might not be sufficient for establishing a gate of an N-channel MOSFET. Specifically, as understood by the present invention a metal electrode that satisfies the threshold voltage design for one type of MOSFET might not satisfy the threshold voltage design for the opposite type of MOSFET or for the same type of MOSFET having a different threshold voltage requirement. As further recognized by the present invention, greater flexibility in designing and implementing MOSFET circuits on ULSI devices would be provided if a single ULSI semiconductor chip were able to incorporate high-K dielectric gate insulators and the metal gate electrodes they entail, and also support more than one threshold voltage. Fortunately, the present invention addresses this consideration.
A method is disclosed for forming at least two types of field effect transistors (FET) on a semiconductor substrate, each type being characterized by a respective threshold voltage. The method includes depositing a high-k gate insulator on the substrate, and depositing a field oxide layer on the gate insulator. Also, the method includes forming at least first and second gate voids in the field oxide layer. A first metal gate electrode is deposited in the first gate void, with the first gate electrode including a first material. Similarly, a second metal gate electrode is deposited in the second gate void, and the second gate electrode includes a second-material. In accordance with the present invention, the second material is different from the first material.
In the preferred embodiment, the gate insulator is characterized by a dielectric constant of at least twenty five (25). The preferred gate insulator can be made of, e.g., titanium(IV)oxide (TiO2) or tantalum(V)oxide (Ta2O5). Also, at least one of the metal gate electrodes is made of material selected from the group of materials including tungsten (W), titanium (Ti), titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN), and molybdenum (Mo).
Isolation trenches preferably are formed in the substrate. The gate voids are established above and between the trenches, that is, each gate void is substantially equidistant from two adjacent trenches.
In a particularly preferred embodiment, the step of forming the gate voids in the field oxide layer includes depositing a first photoresist layer on the field oxide layer, patterning the first photoresist layer to form a first gate window over the field oxide layer, and then removing portions of the field oxide layer below the gate window to thereby establish the first gate void. Likewise, a second photoresist layer is deposited on the field oxide layer and patterned to form a second gate window over the field oxide layer. Portions of the field oxide layer below the second gate window are removed to thereby establish the second gate void. A semiconductor device made in accordance with the method, and a digital processing apparatus incorporating the semiconductor device, are also disclosed.
In another aspect, a method for making a semiconductor device includes depositing a field oxide layer on a semiconductor substrate, and etching away first portions of the field oxide to establish first gate voids. The method also includes filling the first gate voids with a first gate electrode material to establish first field effect transistor (FET) gate electrodes. Moreover, the method includes etching away second portions of the field oxide to establish second gate voids, and then filling the second gate voids with a second gate electrode material to establish second FET gate electrodes. The second gate electrode material is different from the first gate electrode material.
In still another aspect, a semiconductor device includes at least a first field effect transistor (FET) having a first voltage threshold and a second FET having a second voltage threshold. The voltage thresholds are different from each other. As disclosed in detail below, the first FET includes a first gate electrode disposed above a silicon substrate, whereas the second FET includes a second gate electrode disposed above a silicon substrate. A gate insulator is sandwiched between the substrate and gate electrodes. As intended by the present invention, the first gate electrode is made of a first material and the second gate electrode is made of a second material different than the first.
Other features of the present invention are disclosed or apparent in the section entitled xe2x80x9cDETAILED DESCRIPTION OF THE INVENTIONxe2x80x9d.